Apparatus and method for conducting a fischer-tropsch synthesis reaction

ABSTRACT

In order to avoid fouling (precipitation of solid matter on cold surfaces) in heat-exchangers down-stream of the gas outlet of a Fischer-Tropsch reactor, the reactor gas stream containing hydrocarbon products that are solid at lower temperatures is fed into a liquid wash tank. Condensation of heavy oil in the liquid wash tank is effected by feeding an evaporable light oil into the liquid wash tank. Heavy oil is recovered as a bottom product from the liquid wash tank while a gaseous product is taken out of the liquid wash tank as the top product. The light oil is obtained from the wash tank top product.

RELATED APPLICATIONS

The present application is a National Phase entry of PCT Application No. PCT/GB2011/000596, filed Apr. 18, 2011, which claims priority from Great Britain Application No. 1006526.6, filed Apr. 19, 2010, the disclosures of which are hereby incorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to an apparatus and method for conducting a Fischer-Tropsch (“F-T”) synthesis reaction and is particularly concerned with recovering condensable products in the top gas stream from an F-T reactor.

BACKGROUND OF THE INVENTION

The F-T synthesis process is used for converting natural gas, coal or biomass via a syngas unit, into longer chain hydrocarbon waxes. One problem associated with the F-T system is fouling of heat exchangers and other equipment down stream of the reactor gas outlet by the precipitation of higher boiling products in solid form on cold surfaces.

SUMMARY OF THE INVENTION

It is an object of the present invention to address this difficulty.

According to one aspect of the present invention, there is provided a process for recovering condensable products contained within an outlet gas stream from a Fischer-Tropsch reactor, comprising the steps: conveying the reactor gas stream to a condensation and separation unit; subjecting the F-T gases to direct heat exchange in the condensation and separation unit against a stream of evaporable hydrocarbon liquid, thereby cooling the F-T gases; condensing higher boiling components in the F-T gases; separating the condensed higher boiling components from gaseous components; and removing a gaseous fraction and a condensed higher boiling fraction; and in which at least a portion of the evaporable hydrocarbon liquid stream is obtained from subsequent processing of the gaseous fraction obtained in the removing step.

According to another aspect of the present invention, there is provided an apparatus for recovering condensable products contained within an outlet gas stream from a Fischer-Tropsch reactor, comprising a Fischer-Tropsch reactor; a condensation and separation unit; a gas/liquid separator; a line arranged to convey the reactor gas stream to the condensation and separations unit; a condensed higher boiling fraction outlet from the condensation and separation unit; a gaseous fraction outlet line from the condensation and separation unit arranged to convey at least a portion of the gaseous fraction to the gas/liquid separator; and a recycle line arranged to convey evaporable hydrocarbon liquid from the gas/liquid separator to the condensation and separation unit, thereby subjection the reactor gas stream to direct heat exchange in the condensation and separation unit against the stream of evaporable hydrocarbon liquid.

Thus, waxy material which might otherwise foul downstream equipment is removed from the overhead gas stream from the reactor by direct heat exchange with a liquid obtained from the lighter fractions of the gas stream.

Preferably, the reactor gas stream is pre-cooled in order to produce a mixture of gaseous and liquid components before it is conveyed to the condensation and separation unit. Preferably, the pre-cooling comprises indirect heat exchange between the reactor gas stream and a syngas feed stream to the reactor. Preferably, the gaseous fraction obtained from the removing step is subjected to a first separation step in which it is separated into a gas phase, a liquid hydrocarbon phase and an aqueous phase. (It will be understood that there will inevitably be a small amount of water in the hydrocarbon phase and vice-versa.) Preferably, prior to the first separation step, the gaseous fraction is subjected to a cooling step to form a stream of gas and liquid products. Preferably, at least a portion of the liquid hydrocarbon phase from the first separation step constitutes the evaporable hydrocarbon liquid conveyed to the separation unit.

Preferably, the higher boiling fraction removed from the condensation and separation unit is a heavy oil and the liquid hydrocarbon phase from the separation step is a light oil. The products received at the highest temperature (typically in the range 110-150 degrees C.) are referred to as “Heavy Oil” (HO), while product recovered at the lowest temperature (defined by the availability of cooling water) and at typically 30-50 degrees C., are referred to as “Light Oil” (LO).

Preferably, the gas phase from the first separation step is subjected to a second separation step in which it is separated into a gas phase, a liquid hydrocarbon phase and an aqueous phase. Preferably, prior to the second separation step, the gas phase from the first separation step is subjected to a cooling step to form a second stream of gas and liquid products. However, while product recovery has been described as being achieved in two cooling steps, there could be only one, or several such steps.

Preferably, the condensation and separation unit is a liquid wash column. Preferably, the reactor is a slurry bubble column reactor. Preferably, the gas/liquid separator is a liquid flash tank. The apparatus may further include a heat exchanger facilitating indirect heat exchange between the reactor gas stream and a syngas feed stream to the reactor and a second gas/liquid separator arranged to receive a gas stream from the first gas/liquid separator. Preferably, the second gas/liquid separator is a liquid flash tank.

Thus, the present invention may be considered to reside in the use of the liquid wash column where recycled LO is utilized to condense the HO products, while LO is vaporized in a column. The cooling duty to condense HO is scuffed into the cold section with the recycled LO. This design will reduce the partial pressure of heavy components into the cold section of the product recovery section.

The present invention may also be considered to reside in a general non-cryogenic process for the recovery of condensable hydrocarbon components included in a blend of hydrocarbons that may pose a risk of fouling by precipitation of components having a melting point above ambient temperatures (approx. 20° C.) though direct heat-exchange against a stream of evaporable hydrocarbon liquids in a condensation and separation zone and recovery of a liquid fraction of higher-boiling hydrocarbons and a gaseous fraction, from which one or more fractions of lower-boiling hydrocarbons are obtained by subsequent processing of the gaseous fraction. Typically, the condensable hydrocarbon components have boiling points in the ranges corresponding to those of light and heavy oil.

The present invention also extends to a process from conducting an F-T synthesis reaction adopting the aspects of the present invention set out above, in which H₂ and CO are supplied to the F-T reactor and a wax product stream is recovered from the reactor.

Preferably, the reaction is carried out in a slurry bubble column reactor, in which the H₂ and CO are supplied to a slurry in the reactor, the slurry comprising the catalyst in suspension in a liquid including the reaction products of the H₂ and CO, the catalyst being maintained in suspension in the slurry at least partly by the motion of the gas supplied to the slurry. Preferably, the reaction temperature is in the range 190-250° C. and/or the reaction pressure is in the range 10-60 bar. Preferably, the H₂/CO ratio of the gases supplied to the Fischer-Tropsch synthesis reactor is in the range 1.1 to 2.2. Preferably, the superficial gas velocity in the reactor is in the range 5 to 60 cm/s. Preferably, the product of the Fischer-Tropsch synthesis reaction is subsequently subjected to post-processing, such as de-waxing, hydro-isomerisation, hydro-cracking, and combinations of these.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred embodiments of the present invention, reference will now be made to the accompanying Figure, wherein:

FIG. 1 is a simplified flow diagram of a Fischer-Tropsch synthesis installation embodying the present invention

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention may be carried into practice in various ways and one embodiment will now be described by way of example, with reference to accompanying FIG. 1.

The F-T installation comprises a syngas unit 11, an F-T slurry reactor 12, a liquid wash tank or column 13, first and second flash and separator tanks 14, 15 and an optional cryogenic tail gas recovery unit 16.

Natural gas 17, oxygen 18 and steam 19 are fed to the syngas unit 11 where syngas is produced in the conventional manner and leaves as a syngas outlet stream 21. Water is removed in a water recovery stream 22 and directed to a utility system (not shown). The syngas stream 21 is combined with a portion 23 of a gas recycle stream 29 from the tail gas recovery unit 16, and the combined gas stream 24 is heated in a first heat exchanger 25. The combined stream is then fed to the F-T reactor 12 in a line 26 via a second heat exchanger 27 in which it is further heated to about 150° C. by indirect heat exchange with an overhead gas stream 28 leaving the F-T reactor 12 at about 190-250° C. Of the remainder of the gas recycle stream 29, a portion 31 may be purged to a fuel gas system, and a portion 32 is recycled to the syngas unit 11.

The syngas is converted in the F-T reactor 12 to a wide boiling point range of hydrocarbons. Those hydrocarbons that are in the liquid phase at reactor conditions are extracted as liquid and referred to as ‘wax’ (since a substantial portion of the product would normally be solid at ambient temperatures). They are removed from the reactor 12 in a wax product line 33.

To avoid fouling of heat exchangers down stream of the reactor top gas outlet by the precipitation of solid matter on cold surfaces, the overhead gas stream 28, which also contains hydrocarbon products that are solid at lower temperatures is fed to the wash tank 13. After heat exchange with the syngas stream 24 in the heat exchanger 27, the overhead stream 28 is a mixed stream of gas and liquid components. An evaporable light oil stream 34 is fed to the wash tank 13, which condenses heavy oil (which may contain significant amounts of wax components) from the overhead gas stream 28. The heavy oil is removed from the wash tank 13 in a heavy oil outlet stream 35. The feed streams 28, 34 to the wash tank 13 are controlled in such a way that water does not condense in the tank 13.

A gas stream 36 is removed from the wash tank 13 and cooled in a heat exchanger 37 to provide a three-phase stream of gas and liquid products with water. This is fed to the first flash and separator tank 14, from which water is recovered and removed in a water stream 38 and light oil is recovered and removed in a light oil stream 39. A portion of the light oil makes up the evaporable light oil stream 34 that is fed to the wash tank 13; thus the recycled light oil stream acts as a direct cooling medium on the F-T reactor top gas stream in the wash tank 13. The remainder of the light oil is removed in a light oil recovery stream 41.

The uncondensed product from the first flash tank 14 is removed in a gas stream 42 and is cooled in a heat exchanger 43 whereby more liquid products are condensed out and the resulting gas and liquid stream 44 is fed to the second flash and separator tank 15. Water is recovered and removed from the second flash tank 15 in a water stream 45 and light oil is recovered and removed in a light oil recovery stream 46.

Components that remain in the gas phase in the second flash tank 15 are removed as a tail gas stream 47. This may be treated in the tail gas recovery unit 16 to produce a LPG stream 48 and a C5+ stream 49 for upgrading. 

1. A process for recovering condensable products contained within an outlet gas stream from a Fischer-Tropsch reactor, comprising the steps: conveying a Fischer-Tropsh reactor gas stream to a condensation and separation unit; subjecting the F-T gases to direct heat exchange in the condensation and separation unit against a stream of evaporable hydrocarbon liquid, thereby cooling the F-T gases; condensing higher boiling components in the F-T gases; separating the condensed higher boiling components from gaseous components; and removing a gaseous fraction and a condensed higher boiling fraction; wherein at least a portion of the evaporable hydrocarbon liquid stream is obtained from subsequent processing of the gaseous fraction obtained in the removing step.
 2. A process as claimed in claim 1, wherein the reactor gas stream is pre-cooled in order to produce a mixture of gaseous and liquid components before it is conveyed to the condensation and separation unit.
 3. A process as claimed in claim 2, wherein the pre-cooling comprises indirect heat exchange between the reactor gas stream and a syngas feed stream to the reactor.
 4. A process as claimed in claim 1, wherein one or more operating parameters are controlled in order to avoid condensation of water in the condensation and separation unit.
 5. A process as claimed in claim 1, wherein the gaseous fraction obtained from the removing step is subjected to a first separation step wherein the gaseous fraction is separated into a gas phase, a liquid hydrocarbon phase and an aqueous phase.
 6. A process as claimed in claim 5, wherein prior to the first separation step, the gaseous fraction is subjected to a cooling step to form a stream of gas and liquid products.
 7. A process as claimed in claim 5, wherein at least a portion of the liquid hydrocarbon phase from the first separation step constitutes the evaporable hydrocarbon liquid conveyed to the separation unit.
 8. A process as claimed in claim 5, wherein the gas phase from the first separation step is subjected to a second separation step wherein the gas phase is separated into a gas phase, a liquid hydrocarbon phase and an aqueous phase.
 9. A process as claimed in claim 8, wherein prior to the second separation step, the gas phase from the first separation step is subjected to a cooling step to form a second stream of gas and liquid products.
 10. A process for conducting a Fischer-Tropsch synthesis reaction to produce Fischer-Tropsch wax which comprises supplying H₂ and CO to a Fischer-Tropsch reactor, conducting the process set out in claim 1, and removing a wax product stream from the reactor.
 11. A process as claimed in claim 10, wherein the reaction is carried out in a slurry bubble column reactor, and wherein the H₂ and CO are supplied to a slurry in the reactor, the slurry comprising catalyst in suspension in a liquid including the reaction products of the H₂ and CO, the catalyst being maintained in suspension in the slurry at least partly by the motion of the gas supplied to the slurry.
 12. A process as claimed in claim 11, wherein a reaction temperature is in the range of 190-250° C. and a reaction pressure is in the range of 10-60 bar.
 13. A process as claimed in claim 11, wherein a H₂/CO ratio of the gases supplied to the Fischer-Tropsch synthesis reactor is in the range of 1.1 to 2.2.
 14. A process as claimed in claim 11, wherein a superficial gas velocity in the reactor is in the range of 5 to 60 cm/s.
 15. A process as claimed in claim 11 wherein, the product of the Fischer-Tropsch synthesis reaction is subsequently subjected to post-processing.
 16. A process as claimed in claim 15, wherein the post-processing is selected from de-waxing, hydro-isomerisation, hydro-cracking, and combinations thereof.
 17. An apparatus for recovering condensable products contained within an outlet gas stream from a Fischer-Tropsch reactor, comprising a Fischer-Tropsch reactor; a condensation and separation unit; a gas/liquid separator; a line arranged to convey a Firscher-Tropsch reactor gas stream to the condensation and separations unit; a condensed higher boiling fraction outlet from the condensation and separation unit; a gaseous fraction outlet line from the condensation and separation unit arranged to convey at least a portion of the gaseous fraction to the gas/liquid separator; and a recycle line arranged to convey evaporable hydrocarbon liquid from the gas/liquid separator to the condensation and separation unit, thereby subjecting the reactor gas stream to direct heat exchange in the condensation and separation unit against the stream of evaporable hydrocarbon liquid.
 18. The apparatus as claimed in claim 17, wherein the condensation and separation unit is a liquid wash column.
 19. The apparatus as claimed in claim 17, wherein the reactor is a slurry bubble column reactor.
 20. The apparatus as claimed in claim 17, wherein the gas/liquid separator is a liquid flash tank.
 21. The apparatus as claimed in claim 17, further comprising a heat exchanger facilitating indirect heat exchange between the reactor gas stream and a syngas feed stream to the reactor.
 22. The apparatus as claimed in claim 17, further comprising a second gas/liquid separator arranged to receive a gas stream from the first gas/liquid separator.
 23. The apparatus as claimed in claim 22, wherein the second gas/liquid separator is a liquid flash tank. 